1. Field of the Invention
The present invention relates to an optical amplifier.
2. Related Background Art
In WDM (Wavelength Division Multiplexing) transmission systems, in which signal beams consisting of components of a large number of wavelengths are transmitted by introducing these simultaneously into an optical fiber path, optical amplifiers are employed at relay stations etc to make up for transmission losses occurring when these signal beams of multiple wavelengths are transmitted over a long distance. In addition, optical amplifiers are likewise employed to make up transmission losses in passive optical devices (such as for example AWG: Arrayed Wavelength Grating) within relay stations.
Such optical amplifiers are constituted comprising an optical amplification section that amplifies an input signal beam before outputting it and that is obtained by doping with a fluorescent substance capable of excitation by an excitation beam, and an excitation light source device that emits the excitation beam that is supplied to this optical amplification section. For the optical amplification section, Er-doped fiber (EDF: Erbium-Doped fiber) obtained by adding erbium (hereinbelow abbreviated as “Er”) to optical fiber is commonly employed. 1480 nm is commonly used as the wavelength of the excitation beam of this Er-doped fiber; when an excitation beam of this wavelength is supplied to Er-doped fiber, the Er is excited by the optical energy possessed by this excitation beam, causing an inverted distribution condition to be formed by the Er, so that the phenomenon of stimulated emission occurs on input of a signal beam, causing the signal beam to be amplified.
In addition, the excitation light source device comprises a semiconductor laser element that generates the excitation beam. Two types of excitation light source device are known: a type comprising a cooling element for cooling the semiconductor laser element and a type not comprising a cooling element. Of these, an excitation light source device not having a cooling element (hereinbelow called a “non-temperature regulated excitation light source device”) is considered desirable since it can be made of small size and low cost. Such a non-temperature regulated excitation light source device having an excitation beam oscillation wavelength of 1480 nm at 20° C. is previously known.